Thermionic valve circuit



Nov. 11,1941.. w. s. PERCIVAL 2,262,468

THERMIONIG VALVE CIRCUIT V Filed Feb. 23, 1939 3 Sheets-Sheet l 12 111 MODULA TING 1/8 POTENTIAL 1 113 t 3- ,-l- 1 T P INVENTOR W. 8 PERCI VAL ATTORNEY Nov. 11,- 1941. .w. s. PERCIVAL 2,262,468

THERMIONIC VALVE CIRCUIT 7 Filed Feb. 23, 1.939 3 Sheets-Sheet 2 9 INVENTOR 4 J00 BY 5 PERCIVAL 35, 4,5

Sal/leaf aFl ho. 2'1 (av/#5055 1 rams/MW mow 1941- w. s. PERCIVAL THERMIONIC VALVE CIRCUIT Filed Feb. 23, 1959 3 Sheets-Sheet 3 I zr rmz 35 f 4 28 gain-47 T33 T T WT OUTPUT INVENTOR w s. PERC/VAL ATTORNEY OUTPUT Patented Nov, 11, 1941 land, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application'February 23, 1939, Serial No. 258,010 In Great Britain February 24, 1938 11 Claims.

The present invention relates to thermionic valve circuits and has particular reference to thermionic valve switching arrangements. In addition theinvention relates to theme of such thermionic valve switching arrangements in frequency or phase modulated radio transmitters.

It is an object of the present invention to provide means r whereby "a number of thermionic valves can be turned on and off in turn in a predetermined manner. According to the invention a thermionic valve circuit is provided comprising three or more valvesand means for applying switching signals to said valves so as to cause them to assume conducting or non-conducting conditions ina predetermined order, wherein said valves are so arranged that the sum of cathode currents which can bepassed by said valves tends to remain ata constant value so that onthe application'of said switching signals at least one of said valves is prevented from assuming a conducting condition.

According to a feature ofthe present inven tion a frequency modulating system is provided employingathermionic valve circuit operating according to the precedingparagraph wherein each of said valves is so arranged as to tend to generate oscillations of substantially different frequencies and said switching signals comprise modulating potentials whichforce said valves to produce 'oscillationsof a resultant frequency, the.

amplitude f the modulating potentials determining the resultant frequency of the generated oscillations. e I

,Qther features of the invention will appear from the following description and the appended claims.

The invention will be described by way of example with reference to the accompanying drawings in which v v Figure 1 illustrates an embodiment of the in-.- vention,

Figure 2 is an explanatory drawing, and

Figure 3 represents a further embodiment of the invention as applied to a frequency or phase modulating system.

Figures 4, 5 and 6 are modifications of the frequency modulator systems illustrated in Figure 3.

" a potential divider in. parallel with P with two cathode tends to maintain" the current flowing through the valves constant. The resistance R3 may be replaced by any other suitable impedance,

as for example, an inductance. The variable tapping T on the potentiometer P is also connected to the negative side of the source'of potential and to earth. R1 and R2 are equal dropping resistances connected between the positive side of the potential source and the grids of valves V1 and V5 respectively. These two grids areconnected to opposite ends of the potentiometer P. The resistances R4, R5 and Rs constitute together fixed tapping points for the grids of V2 and V4, the centre tap of Rs being earthed. a

The grid of the valveVa is biased by a suitable positive potential as determined by resistance R7 and Rs connected across the anode supply. Also the grids of V2 and V; are supplied with suitable positive potentials by resistances R9 and R10. When tapping T is at the extreme right the biases are such that only valve V1 passes current.

When T is moved towards the left V2 begins to Figure 1 shows a circuit for the control of five valves, the control being affected by a poten- I All five valves V1, V2, etc., have a large pass current, and, as the total current tends to remain constant, this involves a reduction in the current of V1. As T is-moved still further to the 7 left the current of V1 ceases and V3 begins to pass current. When T isat the centre position only V; passes current. Finally with T at its extreme left position only V5 passes current.

In practice it has been found that the values of the resistances mayhave to be adjusted to obtain the required changeover of current, say, from V2 to V3 without V4 passing more than a very small current in the tapping position where V2 ceases to pass current and V3 passes its maximum current. If, however, it is assumed that the variation of anode currentwith grid bias is linear for all potentials to the cut-off potential, then it is possible to calculate the resistances required.

Assuming that the cut-off biasis V0 volts and that the required maximum current for each valve is obtained at zero bias, and that the oath-- I ode potential is large compared with Vo, then a change of V volts in the grid potential of V1 serves to back this valve off completely so that V2 passes its maximum current. This means that the grid bias of V1'has been increased V0 volts with reference to its cathode, while that of V2 has been decreased V0 volts with reference to its cathode, whichis at the same potential as the V1. This implies that the cathode potential has changed by V-Vu volts and the grid potential of V2 has changed by an amount of V-ZVo volts. When V2 is in turn passing zero current and V3 is passing maximum current, the grid potential of V2 will have changed by a total amount V2Vo+2Vo=V volts, since the grid potential of V3 is constant, but the cathode potential will have changed by V volts. Meanwhile, the grid potential of V1 will have changed by 2V0 volts, but the total change of grid bias of V2 must be twice 2V0, that is, 4V0 volts. Thus, 4=4Vc.

These changes are shown in Figure 2, together with the changes for V4 and V5. in this figure that the slope of VgZ and Vg4 is half that of Vgl and V95. Hence R5=2R6 if Re and R are assumed to be large. It will be noticed that when the tapping T is at the extreme right of potentiometer P, the potential on the grid of valve V1 is as indicated at the point K. Since the cathode potential curve Vc intersects the Vgl curve at point K it will be seen that V1 takes the maximum current at this point. When tapping T reaches the point on the potential divider equivalent to M the potential between cathode and grid of V1 is equivalent to LM, which represents V0; in other words, V1 ceases o pass current. At this point the grid cathode potential of V2 is zero and thus V2 passes the has particular application to frequency or phase modulation circuits and in order to explain the operation of such a switching circuit as applied to a frequency modulated radio transmitter, reference should be made to Figure-3 of the accompanying drawings.

. Referring to Figure 3, three screen grid Valves I9, and 92, have their anodes I4, 24 and 96 connected by the condensers I6, 26 and 99 to the points A, B and D on a delay network 30, which comprises series inductances 32 and shunt condensers 33. The delay network 39 is terminated at one end by means of the terminating resistance 3| and an m-derived section 34. The control grids I2, 22 and 94 of the three valves are connected effectively in parallel as regards carrier frequencies by means of condensers I1, 2'! and 9'! to a common point C on the end of the delayv network remote from the terminated end. The screening grids I3, 23 and 95 of the three valves are suitably decoupled and provided with positive potentials by means of resistances 38 and 39 and condenser 40. Resistance I09 forms the grid leak for biasing valve 92. The anode impedances I5, and 98 of the three valves .ll), 20 and 92 are suitably decoupled by means of resistance 14 and condenser 16. A common cathode resistance 18 is employed for all the valves so that the sum of the currents at modulation frequencies taken by the three valves tends to remain substantially constant. The acceptor circuit 79, 80, serves to earth the cathodes for carrier frequencies. The modulation potentials are applied via terminals and r 45 in anti-phase to the grids I2 and 22 of valves land 20 through H. F. chokes l8 and 28, the biasses of the valves being so adjusted that with the application of peak modulating potentials either valve IE] or valve 20 only is operative,

It will be noticed.

whereas with zero applied modulating potentials only valve 92 is operative.

Ihe operation of the circuit can be explained as follows:

Assume that the terminal 35 is made positive and the terminal is made negative so that only valve I0 is conducting. Owing to the pres.- ence of the delay network of length AC between the anode and grid circuits of valve l0 this valve will oscillate at a frequency ii at which the time delay between A and C is equal to the half period, that is, there is a phase change of in the delay network.

If now valve I0 is biased off completely, and only valve 29 is permitted to conduct, then the latter valve will oscillate at a frequency f2 such that the time delay BC is equal to a half period. Similarly, if only valve 92 is conducting, the frequency is at which valve 92 oscillates, is determined by the length CD of the delay network 30. Should both valves l0 and 92 be conducting simultaneously, then if the ratio of the two periods AC and CD is not too great, for example, not more than about 1.5 to 1, then oscillation will occur at a resultant frequency intermediate ]1 and f3, the actual frequency depending on the relative currents taken by the two valves. Thus, the effect of modulation applied between the two terminals 35 and 45 is to make the frequency vary in sympathy between the two extreme limits f1 and f2.

The shunt capacities of the delay network at A, B, C and D may be reduced to allow for the valve capacities and stray capacities in parallel. In the extreme case these capacitances may consist entirely of the valve and stray capacities. If the capacity of the three grids in parallel is too high, then the grids may be separately connected across adjacent sections of the network. The output of the oscillator may be taken from terminals 4| and 42.

As shown, the delay network is terminated by an m-derived section indicated at 34 to prevent undesirable deflection. It is not essential to terminate at the other end of the filter if the grids are connected at this point. If, however, the grids are not connected at, or near, the open end of the delay network, then it is desirable to terminate the network at both ends to prevent reflected oscillations producing on the grids oi the valves additional impulses which may change the frequency of oscillation. In this connection it may be desirable to insert resistances in the grid leads in order to cause the gain of the valves to fall off with frequency in order to prevent oscillations occurring at a frequency in the neighbourhood of the cut-off of the filter.

Although the circuit described in Figure 3 is intended primarily for frequency modulation, it will be obvious that by omitting the feedback path and thus preventing oscillation and by supplying a carrier frequency to the modulated Valves, which then act as radio-frequency amplifiers, the circuits can be adapted for phase modulation. For example, in Figure 3, the lead between C on the delay network and the condenser 21 may be disconnected and a carrier frequency applied to the condenser 21. The phase modulated signals are then derived from 4| and 42.

It will be understood that although the main object of the circuit of Figure 3 is to produce large variations of frequency or phase for frequency or phase modulation, the circuit described can be used for other purposes, such as the remote control of the oscillator frequency for frequency modulation according to the in-' vention, l and 20 aretwo oscillator valves to which modulation frequencies are applied between terminals 35 and 45 in anti-phase to the two grids ,l2 and 22 respectively. [The anodes l4 and 24 of the two valves are joined to a source of positive potential by means of anode resistances l5 and 25. The anode l4 of valve I0 is connected througha blocking condenser l6 to a tapping A on a delay network 30, and similarly the valve anode 24 is connected via condenser 28 to a different tapping B on the network, the latter being terminated at one end by a terminating resistance 3|. The open end G of the delay 4 network is taken to the junction of two condensers I1 and 21, which are connected through modulation frequencies. Thus, the effect of: a

mis-tmatch' between thevalves is reduced to a second order.,

The shunt capacitances of the delay network at A, B and C may be reduced to allow for'the valve capacities and stray, capacities in parallel. In the extreme case those capacitances may consist entirely of the valve and stray capaci ties.

nected across adjacent sections of the network. v As shown, the delay network is terminated by an m-derived section, indicatedat 34, to prevent undesirable reflection. It is not essentialto terminate at the other end of the filter. ifthe grids are connected; at this point. i If, however, 1 the grids are not connected at, or near, the open end of the delay network then it is desirable'to terminate the network at both ends to prevent reflected oscillations giving the grids additionalimresistances I9 and 29, to the respective grids of valves llland20. These condensers function as high impedances for the modulation frequencies and low impedances for the carrierand sideband frequencies. Chokes l8 and 28 are designed to have a low impedance for the modulation frequencies and a high impedance for'the carrier and side-band frequencies. The two valves are biassed by means of the usual cathode resistances 36 and 46 and by-pass condensers 31 and 41. The screen grids l3 and 23 are supplied with positive potentials by means of the potentiometer comprising resistances 38 and 38, condenser 40 being for by-passing purposes. The radio-frequency output can be taken from terminals 4| and42 connected across the open end of the delay network, which comprises series inductances 32 and shunt capacitances 33. The anode re- 7 sistances l5 and should be large compared with the characteristic impedance of the low-pass delay network. r i u The operation of the circuit can be explained as follows: Let the valve 20 be biassed so completely so that only valve III is effective. The valve I0 then oscillates at'a frequency fl, at

pulses which may change the frequencies of 0scillation. .Although resistances I3 and 29 are shown in the grid leads in order to cause the gain to fall off with frequency, any other 5 suitable means may be employed forthis purpose, asfor example, the shunt capacitance at C may be increasedh To increase the mean working frequency, itis desirable to employ a low-passfilter of low char acteristic impedance. It may be found that when this impedance is low enough, that there is insufficient gain for oscillation. In such a case additional amplifying valves may beernployed." The use of such additional valves increases the fiexibility of the arrangement. l

A suitable circuit employing five valves is shown in Figure 5. In this figure, the valves I0 which the time delay between A and C is equal to the half period, 1. e., thereis a phase change of 180 degrees in the delay network. Oscillations would also occur at other higher frequencies if it were not for resistance I9, which causes the gainto fall off with frequency. In practice it fill stead of valve 20, then the latter oscillates at a frequency f2 such that the time delay BC is equal to a half period. If both valves are passing current and the ratio of the two periods AC and BC .is not too great, for example, not more than 1.5:1, then oscillation will occur at an intermediate frequency. Thus, the effect of modulation applied between the two grids is to make the frequency vary in sympathy between the extreme limits f1 and 12.

It is desirable that the valves I0 and 20 be matched. This is not so necessary, however if a common cathode impedance is employed, which is large at the modulation frequencies but small at the frequencies of thecarrier and sidebands.

and, 20 are again arranged to be modulated in A common cathode biassing' ar rangement, comprising resistance 18, condenser 1 l9 and inductance 80, is employed .in this circuit anti-phase.

for reasons already given. 1 Valves 5|] and B0 are the delay filter. with suitable decoupling resistance 14 and condenser 16 is provided for the valves Ill and '20. The anodes of these valves are coupled through condenser 43 and grid resistance 49'to the grid 52 of valve 50. lThe potentiometer comprising resistances 59 and 69 and by-pass condenser 'lll supplies a suitable positive potentialto the screen grids 53 and 63 of valves15|l and 60, which have separate cathode biassing circuits comprising resistances 51 and 61 and condensers 58 and 58. Valves and BI] have separateanode resistances and 65 and a common decoupling resistance 1| and condenser 12. The anode 54 of valve 5|! is coupled via condenser 56 and grid resistance" output can be taken from terminals 4| and 42* connected across the cathode resistance 9|. The cathode 82 is connected to a tapping C on the network, which tapping is coincident with the tapping B. It has been found with this circuit that there'is sufficient phase change in theam-' plifier to give oscillation without an additional If the capacityof the two grids in parallel is too high then the grids may be separatelycon delay network between B and C. Thus, the frequency of oscillation depends substantially on the phase change dueto the valve and stray capacities shunting the anode and grid resistances. The delay section from A to BC is, however in circuit when valve I only is operative. The frequency obtainedwith valve I0 alone is, therefore, the

minimum frequency and that with valve alone the maximum frequency to the minimum frequency obtainable and with ratiosabove 1.5:1 there is a tendency to serious amplitude modulation; This limitation can be overcome by the use of more than two modulated valves as shown in Figure 3.

The circuit shown in Figure 6 illustrates a further method of frequency modulation. In Figure 6 a delay network is shown with series inductances 32 and shunt capacitances 33 and terminated at 34, 3|. At the end of the delay network remote from 3| is connected a negative resistance l0! across which the output terminals 4| and 42 are connected. Diode valves I00, II I and I I4 are connected across'the delay network at separate tapping points, the cathodes I09, H2 and H5 of which are connected to terminal 42 by'means of condensers I20, I2I and I22. The modulation frequencies are applied to terminals and. 45, and through the series isolating resistances H1, H8 and H9 to the cathodes I09, H2 and H5. Bias batteries I23 and I24 are provided for the diodes III and I I4.

If the delay network is of lower characteristic impedance than the negative resistance, which may be a dynatron, and it is correctly terminated at the end remote from the negative resistance then'oscillation will not take place, assuming that thediodes are out of circuit. If the delay network is shorted at the remote end then oscillation A will take place at a frequency such that the delay from the negative resistance end of the filter to the shorted point is equal to a quarter of a period. Now, if a'number of diodes are connected across different points on the delay network as shown and if the slope resistance of each diode is low compared with the characteristic impedance of the filter then the frequency of oscillation will be determined almost entirely by the position of the first diode I00 nearest to the negative resistance. As the first diode is gradually biassed oiT the frequency of oscillation is determined mainly by the position of the second diode I I I. As this second diode is in turn biassed off the frequency of oscillation will be determined chiefly by the position of the third diode H4 and so on, if further diodes are employed. Thus, by applying modulation so that the diodes are biased off successively frequency modulation is obtained. By employing a sufliciently long delay network and suificient diodes there is theoretically no limit to the range of frequency modulation that may be obtained. In the circuit shown the cathode of the second diode III is biassed more than diode I08 and similarly diode H4 is biassed more than diode I I I, in order that the modulation shall bias the valves off successively.

A number of modifications of the arrangement will occur to one skilled in the art. The essential feature of the circuit described above is the 1 use of one or more valves to vary the eifective length of a delay network and thus change the frequency of oscillation. By substituting the output of an amplifying valve for the dynatron, and applying a signal of fixed frequency, phase modulation can be obtained.

I claim:

1. In an electrical system, a plurality ofvalves each having a control electrode and output electrodes including a cathode, direct-current paths including said output electrodes for supplying anode-cathode direct-current for all of said valves, means for maintaining the sum of the anode-cathode direct current drawn by all of said valves substantially constant, a source of variable control potentials connected with the control electrodes of said valves for controlling the conductivity of said valves in a pro-determined sequence by varying the control electrode potentials whereby certain of said valves periodically drawn more current than other of said valves and vice versa, and an output circuit connected with said output electrodes.

2. A system as recited in claim 1 wherein said cathodes are tied together and said direct-current paths including said output electrodes have a common portion, and wherein said means for maintaining the sum of the cathode-anode current substantially constant is in said common portion.

3. In an electrical system, a plurality of valves each having a control electrode and ouput electrodes including a cathode, a potentiometer resistance energized by direct-current voltage, means for deriving from said resistance variable control potentials, said means comprising connections between the control electrode of each of said valves and a different point on said resistance, a connection tying the cathodes of said valves together and a connection between said last connection and a movable point on said resistance, whereby different biases are supplied to said control electrodes and certain of said valves are conductive and others less conductive and adjustment of said movable point on said resistance increases the conductivity of certain of said valves and decreases the conductivity of other of said valves, means in said last named connection for maintaining substantially constant the anode-cathode current drawn by said valves and an output circuit connected with the output electrodes of said valves.

4. In a signalling sytsem, a plurality of valves each having a control electrode, an anode electrode, and a cathode, a line comprising series and shunt reactances, means connecting the control electrodes of said valves to a point on said line, means connecting the anode of one of said valves to a second point on said line, means connecting the anode of another of said valves to a point on said line intermediate said first two points, a direct-current circuit including the anode and cathode of each of said valves, means in said direct-current circuit for maintaining substantially constant the anode-cathode current drawn by said valves, means for modulating the impedances of a pair of said valves in phase opposition, and an output circuit connected with the output electrodes of said valves.

5. In a frequency modulation system a plurality of valves each having a control electrode and output electrodes including a cathode, a source of current for supplying anode-cathode current for all of said valves, means in the anodecathode-direct-current path of between each of said valves and said source for maintaining the sum of the anode-cathode current drawn by all of said valves substantially constant, oscillation generating circuits tuned to different frequencies regeneratively connected with the control electrodes of each of said valves for the production of oscillations of different frequency, and means for modulating the impedances of said valves at signal frequency to cause said valves to pro- ,nected with the control electrodes of certain of said valves for controlling the conductivity of said valves in a pre-determined sequence characteristic of the modulating potentials, whereby certain of said valves periodically draw more current than other of saidvalves and vice versa, a time delay circuit wherein wave energy to be modulated is caused to flow, means connecting the anodes of saidvalves to spaced points onsaid time delay circuit, and an output circuit 1 connected to points on said time delay circuit.

7. In a frequency modulation system, a plurality of valves each having a control electrode,

an anode, and a cathode, a source of direct current in a circuit including the cathodeand anode of all of said valves, means in said direct-current circuit for maintaining the sum of the anodecathode currents drawn by all of said valves substantially constant, a source of modulating v potentials connected with the control electrodes of certain of said valves for controlling the conductivity of said valves in a pre-determined sequence characteristic of the modulating potentials, whereby certain of said valves periodically draw more current than other of said valves and vice versa, a time delay circuit comprising series and shunt reactances, connections between a point on said time delay circuit and each of said control electrodes, connections between spaced points on said time delay circuit and the anodes of said valves, and output terminals connected with said time delay circuit.

8. In a frequency modulation system, a plurality of valves each having a control electrode and output electrodes including a anode-cathode,

, a source of current connected in a cathodedirect current circuit including the cathodes of all of said valves, means in said direct-current circuit for maintaining the sum of the anodecathode currents drawn. by all of said valves substantially constant, oscillation generating circuits of different periodicity connected between the electrodes of each of said valves, said valves each tending to generate oscillations of a different frequency, a source of modulating poten tials connected with the control electrodes of certain of said valves for controlling the'frequency of operation thereof in accordance with the modulating potentials, and a common output circuit connected with the output electrodes of all of said valves.

9. In a frequency modulation system, a plurality of valves each having a control electrode and output electrodes including a anode-cathode, a source of current connected in a cathodedirect-current circuit including the cathodes of all of said valves, means in said direct-current circuit for maintaining the sum of the anode-t cathode currents drawn by all of said valves substantially constant, oscillation generating circuits comprising a transmission line terminated,

by a matching impedance having spaced points thereon connected between the electrodes of each of said valves, whereby said valves each tend to generate oscillations of a different frequency, a source of modulating potentials connected with the control electrodes of certain of said'valves for controlling'the frequency of operation thered of, in accordance with the modulating potentials,

and a common output circuit connected withthe output electrodes of all of said valves.

10. In an electrical system a plurality of electron discharge devices each having an anode and a cathode, direct-currentpaths connecting the anode and cathodes of each of said devices in a direct-current circuit, an impedance in a portion of said direct-current circuit common to all of said paths for maintaining the sum of the anode-cathode direct currentdrawn byall of said devices substantially constant, means for controlling the conductivity of said devices in a predetermined sequence by varying the impedance between the anode and cathode thereof in accordance with controlling potentials whereby certain of said devices periodically draw more current than other of said devices and vice versa,

and means for deriving output currents from v the anodes and cathodes of said devices.

11. In an electrical system a plurality of electron discharge. devices each having electrodes including an anode and a cathode, a time delay network comprising series and shunt reactances, a connection between the cathode of each device and a point on said network, connections between other corresponding electrodes of each of said devices and other and differentpoints on said line, means for controlling the conductivity of said devices in a predetermined sequence by varying the impedance between the anode and cathode thereof in accordance with controlling potentials whereby certain of said devices periodically draw more current than other of said devices and vice versa, and means for deriving output currents from the anodes and cathodes of said devices.

WILLIAM SPENCER PERCIVAL. 

